This invention relates generally to the field of medical ventilators. More particularly, it relates to a system for controlling the flow of inspiratory gas in a volume-cycled, pressure-limited ventilator.
Volume-cycled, pressure-limited ventilators (commonly called "volume" ventilators), have become well-established for life support and respiratory therapy, particularly for adults. While the volume ventilator has assumed numerous forms, in general it operates by providing a predetermined volume of respiratory gas (air or oxygen-enriched air, typically with added water vapor) to the patient during the inspiratory phase of each breathing cycle. Specifically, the volume ventilator delivers the gas in accordance with a predetermined flow rate function, wherein the delivered flow rate is integrated over time until a predetermined volume is delivered.
A typical volume ventilator includes a flow control valve that is electronically or pneumatically actuated to produce an instantaneous flow rate throughout the inspiratory phase that corresponds with a preselected flow rate function, as set by the operator. An example of such a flow control system is disclosed in U.S. Pat. No. 4,527,557--DeVries et al., assigned to the assignee of the invention disclosed and claimed herein. The system disclosed in the DeVries et al. patent comprises a flow control valve actuated by a stepper motor that is controlled by a control signal generated by a microcomputer. The microcomputer generates the control signal by comparing an instantaneous flow rate signal produced by a flow transducer with a flow rate value required by a flow rate function stored in the microcomputer's memory. Other volume ventilator systems using a signal from a flow sensor to actuate a flow control valve are disclosed in the following U.S. Pat. Nos.: 3,923,056--Bingmann et al.; 3,961,627--Ernst et al.; 3,972,327--Ernst et al.; and 4,928,684--Breitenfelder et al.
The system disclosed in the above-mentioned DeVries et al. patent exemplifies the use of a real time flow rate-indicative signal, generated by a flow transducer, as a feedback signal, wherein the instantaneously sensed flow rate is the parameter whose value is compared with the stored nominal value to generate the stepper motor control signal. The use of a closed-loop feedback system, including a stepper motor under the command of a microprocessor, to operate the flow control valve allows the system to achieve a relatively high degree of precision over a wide range of flow rates, with the ability to accommodate a wide variety of flow rate patterns.
While the system described above has provided highly satisfactory levels of performance, there has been a desire to improve responsiveness and reliability beyond the limitations inherent in state-of-the-art flow transducers. For example, instead of sensing flow rate directly, a value for the instantaneous flow rate can be calculated by measuring the pressure drop across a flow orifice of known area. State-of-the-art pressure transducers can achieve high levels of accuracy and reliability, and a variety of means can be used to determine the size of a variable orifice, either directly or indirectly, with precision. With state-of-the-art high speed microprocessors, a highly precise value for the instantaneous flow rate can be obtained in real time or near real time.
The general method of measuring a fluid flow rate as a function of the sensed pressure differential across an orifice is well-known in the fluid metering art, as exemplified by the following U.S. Pat. Nos.: 3,055,389--Brunner; Re. 29,383--Gallatin et al.; and 4,277,832--Wong. This general method has also been employed to measure air flow in air conditioning and room ventilation ducts, as shown in U.S. Pat. No. 4,026,321 Kahoe et al. and U.S. Pat. No. 4,796,651--Ginn et al.
Medical ventilators have likewise employed flow control systems in which an instantaneous flow rate value is calculated from a measured value for pressure and a known or measured value for flow orifice size. Examples of such ventilators are disclosed in U.S. Pat. No. 4,637,385--Rusz and U.S. Pat. No. 4,883,051--Westenkow et al.
As far as is known, despite the desire for ever-increasing reliability, responsiveness, and flexibility in flow control capability in volume ventilators, the prior art has not contemplated the precision actuation of a ventilator flow control valve using a stepper motor under the command of a microprocessor, wherein the microprocessor transmits a correction or control signal to the stepper motor in response to signals indicative of (a) the varying differential pressure across a variable flow orifice in the valve, and (b) the effective flow area of the valve orifice.